fix: skip symlink cycles during pack instead of failing with ELOOP

[gagan-53]

Fixes #292.

Problem

When an extension directory contains a symlink cycle (e.g. ln -s . selfloop), mcpb pack aborts with a raw OS error and produces no archive:

ERROR: Archive error: ELOOP: too many symbolic links encountered, stat '.../demo-ext/selfloop/selfloop/.../selfloop'

getAllFiles and getAllFilesWithCount in src/node/files.ts walk the tree with statSync (which follows symlinks) and recurse into anything reported as a directory, with no tracking of already-visited paths, so a cyclic link descends until the OS raises ELOOP.

Fix

Track the real paths (realpathSync) of the directories on the current traversal path. Before recursing into a directory, resolve its real path; if it is already on the traversal path, emit a warning naming the offending path and skip it:

Warning: Symlink cycle detected, skipping: selfloop

Packing then continues and succeeds. Behavior for everything else is unchanged:

• Non-cyclic symlinks (files and directories) are still followed and their targets packed, as before — important for layouts that rely on symlinked directories.
• Only the traversal path is tracked (entries are removed after recursion returns), so two independent links to the same directory are still both packed, as before.
• The new visitedRealPaths parameter is optional and trailing on both exported functions, so the public API is backward compatible.

Testing

• New test/symlink-cycle.test.ts: self-referential symlink, mutually recursive symlinks (A→B, B→A), a non-cyclic symlink still being followed, and coverage of both getAllFiles and getAllFilesWithCount. The cycle tests fail with ELOOP without the fix. Symlinks are created with type junction (works unprivileged on Windows; ignored elsewhere) and the tests no-op if the environment cannot create symlinks.
• yarn test: 129 passed, 9 suites.
• yarn lint: clean.
• Manual repro from the issue: before — ERROR: Archive error: ELOOP: ..., exit 1, no archive; after — warning + successful pack, archive written, exit 0.